1. Field of the Invention
The present invention relates to a CRT-type display device, and more particularly, to a display device having a shield case (inner cabinet) which provides shielding against electromagnetic interference (EMI) and contains a fire which may break out in the interior of the shield case (inner cabinet).
2. Description of the Related Art
Referring to FIG. 1, a typical CRT-type display device is largely comprised of: a cathode ray tube (CRT) 20 by which an image is displayed according to a display signal transmitted along with an electric energy; a circuit board 30 mounted on the rear side of the CRT 20, to control electrical power, the display signal and related circuitry; a front case 10 to which the CRT 20 is mounted; and a rear case 50 which is combined with the front case 10 to encase the interior of the display device. The circuit board 30 is mounted on a circuit board frame 40 which is secured to the front case 10 or the CRT 20. A bottom frame 90 is mounted to the bottom of the circuit board frame 40, and a stand (not shown) is generally provided under the bottom frame 90 for support of the entire display device.
In order to provide EMI shielding for such a display device, a shield case 60 is attached to the rear of the CRT 20 and the top of the circuit board 30 to thereby cover these areas and thus minimize the radiation of electromagnetic waves generated by the display device as well as to protect the display device from external electromagnetic interference. As a rule, display devices having screens smaller than fourteen inches do not employ this shielding.
As shown in FIGS. 2A and 2B, the shield case 60 is formed of a top plate 60a, a pair of lateral plates 60b formed on two sides of the top plate 60a, and a rear plate 60c forming the rear surface of the top plate 60a. The shield case 60 is generally cut out of a flat piece of sheet metal to form panel-like sections and then stamped into the desired shape. Therefore, when assembled, the shield case 60 tends to have gaps 65 where the respective plates meet and where complete enclosure is considered unnecessary since the shield case 60 is primarily for EMI shielding purposes only. A plurality of cooling holes 61 are formed throughout many surfaces of the shield case 60 at predetermined locations, to reduce weight and to allow air to circulate within the interior of the shield case 60, while maintaining adequate shielding effects.
As a significant feature of the shield case 60, an anode cap shield 62 is provided in a surface of the top plate 60a and protrudes upwardly from the surface to a fixed height determined by the presence of an anode cable 32 connecting an anode cap 31 through which high voltage passes to the circuit board 30 to the CRT 20. Since there is a potential for electrical contact between the anode cap shield 62 and the high-voltage components, an insulating sheet 80 may be installed using notched slots 81 on the underside of the anode cap shield 62 to promote safety.
The anode cap shield 62 is formed of a separately cut and stamped panel attached to the top plate 60a, resulting in the formation of openings 63 on either side thereof. Even though the openings 63 may be large and a measurable degree of electromagnetic emission escapes through the openings, the shield case 60 can still meet the prescribed shielding requirements.
The entire assembly shown in FIG. 2B is fixed to adjacent coupling components, e.g., the CRT 20 or CRT bracket (not shown) and the circuit board frame 40, using coupling means such as a hook and groove structure or sheet metal screws. In so coupling, many open portions 64 are present due to weight reduction goals and component access considerations.
Although EMI requirements are met by installing the shielding case 60 constructed as above in a CRT-type display device, in order to meet fire safety standards such as those set by Underwriter's Laboratories (UL) (e.g., UL Standard 1418 (1992), Standard for Safety for Cathode Ray Tubes), the rear case 50, and sometimes the front case 10, must be manufactured from fireproof material. That is, if a fire should break out within the interior of the display device, the shielding case 60 offers no protection from the possibility that the fire should spread outward from the device and into the surrounding room. Thus, the rear case 50 should be designed to contain an accidental fire.
In manufacturing and adopting cases made of fireproof material, there is no particular structural problem. However, there is a drawback in that the case, being made of fireproof material, is costly. Besides fireproof material, UL safety requirements stipulate additional measures, for example, the size, shape, position and number of outer cabinet cooling holes. It should therefore be appreciated that when UL requirements are placed on the design of the case, which constitutes the overall appearance of the product, the fundamental design of the entire product, as it appears to the consumer, is restricted.
In addition, the extensive use of fireproof material presents an environmental hazard and is a continual draw on natural resources.